Either way, its a bad deal, generic PC components will require that your laptop frame is relatively large, defeating much of the purpose of a laptop. Trying to fit components into your laptop that were specifically designed for other frames may prove largely unsuccesful and costly. And buying propietary components specifically intended to fit into your frame only is, needless to say, going to be costly in and of itself.
Actually, from my experience Barebone laptops come with everything that doesn't have a size standard. They commonly come with the motherboard, the GPU, and the optical drive. Things it doesn't come with is the CPU, the RAM, and the HDD. Basically the items that have a standard form factor throughout laptops. It's almost exactly like a barebone PC, the exception being that, as you said, the parts that are included are frequently custom to the casing.
I personally can't wait to buy an ASUS C90S for my E6600 when I upgrade to an E6850. I just love the idea of a barebone laptop that takes a desktop processor.